1. Field of the Invention
The present invention relates to a method of detecting conical folding in geological formations and, more particularly, to such a method which utilizes an existing dipmeter survey in conjunction with one or more synthetic dip plots.
2. Setting of the Invention
The existence of conical folding of geological formations has been debated within the geological community for many years; however, it is now generally accepted that subterranean geological formations can be folded and folded in such a manner to either be cylindrical or conical in general shape. One way to visualize conical folding is a plurality of cups that have been nested with an essentially horizontal plane passing through the center axis of each of the cups.
During the exploration for oil and gas, wellbores are drilled through the geological formations, and oftentimes various forms of logging tools are used to determine certain parameters of the geological formations. One such logging method is called a dipmeter survey, which is well known to those skilled in the art. A dipmeter survey provides an indication of the inclination and direction of each geological formation. As dipmeter surveys are becoming more widely used in the exploration for oil and gas, it has been observed that certain distinctive patterns appear on dipmeter surveys. Upon later analysis of these distinctive patterns, it has been found that some of these can be related to, i.e., are caused by, cylindrical or conical folding of the geological formations. Due to the migration effect of the oil and gas, it is desired for the wellbore to penetrate the geological formations at the apex or uppermost portion of each of the geological formations, and a dipmeter survey assists in the location of such desired apex.
With conical folding, the location of this apex is uncertain. There is a need within the industry for a method of determining the existence of conical folding within geological formations from a dipmeter survey such that one skilled in the art, upon review of a dipmeter survey, can determine the existence of conical folding with greater confidence.
A good description of conical folding and the resulting dipmeter tangent diagrams are discussed in "Structural Uses of Tangent Diagrams" by Bengtson in Geology, Vol. 8, p. 599-602, December 1980, which is incorporated herein by reference. Within the Bengtson article, cylindrical and conical folding are described, as well as tangent diagrams of the dip data are shown. In Bengston conical folding is assumed and the tangent plots are provided. There is no disclosure or suggestion within Bengston of first obtaining a dipmeter plot and then producing synthetic dipmeter plots and comparing the two to determine if indeed conical folidng is present. Further, there is no disclosure or suggestion within Bengston of using any other type of dipmeter plot other than the tangent diagrams.